The technical field of this invention is communication systems, particularly communication systems as applied in digital wireless telephone handsets.
In a digital telephone handset, the speaker""s voice is converted to an electric current in a microphone. This current is then amplified in an analog preamplifier, and converted to a digital representation using an analog to digital converter. The output of the analog to digital converter is a pulse code modulated signal that in a wireless telephone application will typically have a dynamic range of 13 bits. This digital signal is then passed through a digital filter to remove aliasing that may be generated in the analog to digital converter, and is then passed on to a Digital Signal Processor for further processing. In order to compensate for the manufacturing tolerances of the various components in this chain, the gain of the analog preamplifier and the digital filter is programmable. During the manufacturing process, the gains of the various amplifiers are adjusted to balance the overall gain of the system to the design requirement, usually by adjusting a PGA (Programmable Gate Array). Once this adjustment is made, the gains become fixed, and can no longer be changed.
When the phone is used in a location with a high background noise, a situation is encountered known as the xe2x80x9cloud talker environmentxe2x80x9d. The combination of the high background noise and the louder than normal speech of the user will generate an abnormally large signal from the microphone and preamplifier. This may then exceed the dynamic range capability of the analog to digital converter and the digital filter, resulting in clipping and distortion. In an extreme case, this clipping may result in the PCM signal becoming fully saturated, in which case all of the intelligence content of the signal is lost. In a less serious case, the clipping will result in a distortion of the voice signal. The Digital Signal Processor may attempt to correct the distortion, but it is usually not possible to do that in a satisfactory manner since the damage is done by the time the DSP receives the signal. In the digital phones available today, this is a major problem that seriously limits the usefulness of the phone in a noisy environment.
The present invention provides an active Automatic Gain Control (AGC) system that uses the processing power available in the Digital Signal Processor to detect the distortion caused by the clipping in the ADC and the digital filter, and then adjusts the variable gain components to maintain the signal level below the clipping threshold. While the current invention shows this applied in a digital wireless telephone, the same problem exists in voice codecs in general, and the solution applies wherever voice is digitized in a high background noise environment.
While Automatic Gain Control circuits are know in the art and are commonly used in analog communication systems, they are difficult to implement in a digital system. The present invention makes the implementation practical by using the processing power available in the Digital Signal Processor to recognize the distortion, and to apply the required correction.